The mean daily and annual discharges for 27 observation stations on 19 rivers in the Han River basin (Korea) were analyzed using data collected over periods of 10 to 23 years, and the corresponding bankfull discharges were estimated using stage-discharge curves, based on the water stage identified by the geomorphic bankfull indicators for the same sites. The characteristics of the geomorphic bankfull indicators were also investigated to assess their utility in estimating the bankfull discharge of a target basin. The correlation between the estimated bankfull discharge and the observed discharge over several years was examined for the same observation stations, using the stage-discharge curve. The bankfull discharge is defined as the seven-day mean discharge, and its relations with the mean monthly and annual discharges is defined accordingly. The hydraulic geometry relationships for drainage area and bankfull discharge were analyzed based on the regime theory, and a newly developed equation is proposed for estimating the bankfull discharge using significant hydraulic geometric variables with high determinants.

The stream flow generation method is necessary for predicting yearly bed change at an ungauged stream in Monsoon region where there is no hydrologic and hydraulic information. This study developed the stream flow generation method of daily mean flow for each month over a year for bed change simulation at an ungauged stream. The hydraulic geometries of cross-sections and the corresponding bankfull indicators of the Byeongseong river of 4 km reach were analyzed to estimate the bankfull discharge. The estimated bankfull discharge of the target reach was 77.50 m³/s, and the total annual discharge estimated 3720 m³/s through the correlation equation with the bankfull discharge. The measured total annual discharge of the Byeongseong river was 3887.30 m³/s, which is greater by 167.30 m³/s of 4.3% relative error. The volume and bed changes over a year by the Center for Computational Hydroscience and Engineering Two-Dimension (CCHE2D) model simulated using the measured discharge during 2013 and 2014 coincided with the surveyed in the same period. Estimated total annual discharge was used for the scenarios of stream flow generation. The generated stream flow using the flow apportioned to each month on the basis of the flow percentage in an adjacent stream simulated the river bed most appropriately. The generated stream flow using the flow based on the monthly rainfall percentage of the rainfall station in the target stream basin also simulated river bed well, which is confirmed as an alternative. Quantitatively, the root mean square error (RMSE), mean bias error (MBE), and mean absolute percentage error (MAPE) in-depth change of thalweg between the measured and the simulated were found to be 0.25 m, 0.04 m, and 0.44%, respectively. The result of the simulated cross-sectional river bed change for target reach coincided well with the surveyed. The proposed method is highly applicable to generate the stream flow for analyzing the yearly bed change at an ungauged stream in Monsoon region.